In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in ¼ cup of all-purpose flour and cook for about 1 minute until it begins to bubble. Gradually add 3 cups of milk while whisking continuously until the mixture thickens, approximately 5-7 minutes. Stir in a pinch of nutmeg, salt, and pepper, then set the sauce aside to cool slightly.

Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add 1 chopped onion and 2 minced garlic cloves, sautéing until translucent, about 3 minutes. Then, incorporate 1 chopped bell pepper and 1 diced zucchini, cooking until tender—approximately 5-7 minutes. Finally, stir in 2 cups of thawed and drained frozen spinach, seasoning with salt and pepper. Let this filling cool.

In a mixing bowl, combine 15 ounces of ricotta cheese, ½ cup of grated Parmesan, and 1 beaten egg. Stir until well blended, then gently fold in the cooled vegetable mixture until fully integrated.

While the cheese filling cools, boil a large pot of salted water. Cook 9 lasagna noodles according to package instructions until al dente, about 8-10 minutes. Drain them and immediately rinse with cold water to stop the cooking process. Lay them flat on a clean kitchen towel to prevent sticking.

Preheat your oven to 375°F (190°C). In a 9x13 inch baking dish, spread a ladle of white sauce on the bottom. Place three lasagna noodles horizontally, covering the sauce. Next, layer half of the vegetable cheese filling, followed by a third of the mozzarella cheese. Repeat the layers: sauce, noodles, filling, and mozzarella, finishing with noodles and topping off with the remaining white sauce and mozzarella.

Cover the assembled Loaded Veggie White Lasagna tightly with aluminum foil and bake for 25 minutes. Afterward, remove the foil and bake for an additional 20-25 minutes, or until the top is golden and bubbly. Once baked, let the lasagna cool for about 10 minutes before slicing and serving.